Solar wind particles solve lunar mystery

Trace chemicals ejected from the Sun and collected by NASA’s Genesis mission have solved a long-standing lunar mystery that threatened to rewrite our understanding of how the Sun evolved.

For the last 4 billion years, energetic solar particles have bombarded the Moon. But studies of these particles in rocks brought back by the Apollo astronauts have mystified scientists.

That is because the ratio of two isotopes of neon have varied according to depth in the rocks, with comparatively more neon-22 than neon-20 at lower depths. That suggested that counter to theory, the Sun had once been significantly more active than it is today, shooting out higher energy particles that could travel farther into the rocks.

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Now, Ansgar Grimberg at the Swiss Federal Institute of Technology (ETH) in Zurich, and colleagues have resolved the conundrum.

They used nitric acid to strip away layers of a specially made metallic glass that had been exposed to the solar wind for 27 months on the Genesis spacecraft, which crashed to Earth in 2004.

When they measured the neon distribution in the exposed solar wind samples, they found the top layer had considerably higher proportions of neon-20 than observed in the lunar samples, while the underlying layers were similar to those seen in the Moon rocks.

That suggests that erosion from micrometeorites and space particles removed some of the original neon from the top surface of all lunar rocks.

More importantly, it also shows that the solar wind alone – not any extra activity on the Sun – can explain the puzzling neon variations in the Moon rocks, with the heavier neon-22 simply implanting itself more deeply than neon-20.